Scientists Say San Onofre Has Flaw

Federal regulators are looking into potential flaws at the San Onofre nuclear power station and 67 other atomic plants across the nation that could cripple or knock out a reactor’s emergency core cooling system during an accident.

The defect has prompted the Union of Concerned Scientists to warn that the San Onofre station near San Clemente is among the most likely to suffer reactor damage and possible meltdowns because of the defect.

“San Onofre is one of the more risky in the nation, but they all need to be fixed,” said David Lochbaum, a nuclear engineer who is looking into the problem on behalf of the Washington-based environmental group.

Officials for Southern California Edison, which operates two reactors at San Onofre, say the chance of an accident is remote and steps are being taken to fix the design flaw.

The problem involves the possibility that sump screens that protect emergency pumps might become clogged with debris during an accident, restricting or preventing vital cooling water from reaching the reactor.

There has never been a sump failure in the United States.

In June, the U.S. Nuclear Regulatory Commission issued a bulletin to the operators of 68 of the nation’s 69 pressurized-water reactors, requesting information about how they were dealing with the risk of sump failure. There are 103 nuclear power plants in the U.S. The agency plans to have the problems resolved by 2007.

Last February, the commission halted operations at the Davis-Besse nuclear reactor in Ohio after bits of insulation, paint chips and other debris were found in the reactor containment structure. Officials said the conditions were of “substantial importance to safety.”

Similar problems were found in 1996 in the nation’s 34 boiling-water reactors, but the defects were corrected by installing larger sump screens.

In boiling-water reactors, water is allowed to boil in the reactor core before the steam enters the power station’s turbines. In pressurized-water reactors, water is brought to a boil in steam generators after being heated under pressure in the reactor.

“We have been following this issue for some time, perhaps since the 1980s,” said David McIntyre, an NRC spokesman. “Now, the concern is on pressurized-water reactors.”

McIntyre said the June request was based on recent research by the Los Alamos National Laboratory in New Mexico.

The studies concluded that water escaping under high pressure through a broken pipe or pressurized vessel could scour insulation, paint and protective coatings from pipes, walls and machinery.

The debris, researchers said, could then collect on sump screens and block or restrict the flow of water to the reactor’s emergency pumps. If the reactor were not cooled properly, it could overheat and release radioactive material into the containment building and potentially into the atmosphere.

The Los Alamos studies indicated that San Onofre was among the 10 power plants in the nation most likely to have a sump failure if an accident occurred.

The Union of Concerned Scientists, an alliance of 60,000 citizens and scientists, contends the federal studies reveal a serious safety problem that needs to be corrected before the commission’s 2007 deadline. It released a research paper calling attention to the defect in August and petitioned federal regulators Sept. 8 to shut down the Indian Point power plant in New York until its flaw was corrected.

Citing federal data, Lochbaum concluded that 77% of all pressurized-water reactors were very likely to have sump failures if there were a large breach of the emergency-cooling system. Such failures, he said, would occur in 36% of reactors during small accidents. Overall, it contends there is a 1 in 3 chance that one of the 68 pressurized-water reactors will experience reactor-core damage before the NRC deadline for sump repairs.

Federal regulators accuse the group of overstating the chances of such an accident. Other emergency steps, they say, can be taken to minimize the possibility of the sumps becoming a problem, such as backup sources of cooling water. Consequently, McIntyre said, the chance of a sump blockage during an accident is less than 1 in 100 when “more realistic” factors and assumptions are considered.

But Lochbaum said some of the alternative emergency measures have not yet been proved or have not been installed in many of the nuclear plants in question. “You could end up trading one problem for another,” Lochbaum said. “If you can show that it works, fine. But don’t take shortcuts.”

The chance of sump failure can be reduced by minimizing insulation, judicious use of paints and coatings, better maintenance and using larger screens that are less likely to clog.

Edison officials said some of these steps have been taken at San Onofre. In addition, they said, there are improved inspections of sump screens and pumps, and four sources of cooling water are available for emergencies. “Given the way we maintain the pumps and the cleanliness of our containment structure, these events would not occur,” said Ray Golden, an Edison spokesman. “The scenario of a pipe break and pump failure is extremely remote.”